1. Field of the Invention
The invention relates to a turbojet and in particular a sealing device for a turbojet oil enclosure.
2. Description of the Related Art
A turboshaft for an aircraft generally comprises, arranged in the direction of the gas flow: a fan, one or more compressor stages, for example a low-pressure compressor and a high-pressure compressor, a combustion chamber, one or more turbine stages, for example a high-pressure turbine and a low-pressure turbine, and a gas exhaust nozzle. Each compressor may be associated with a turbine, the two elements being linked by a shaft, thereby forming, for example, a high-pressure core and a low-pressure core.
Turbojets generally have, substantially around the upstream extremity of the high-pressure core, an “upstream enclosure” containing bearing and gear members. They also generally have, substantially around the downstream extremity of the high-pressure core, a “downstream enclosure” containing oil-lubricated bearing and gear members. The oil, projected by these rotary parts , forms a mist (or suspension) of suspended droplets within the enclosures. Furthermore, they are traversed by a gas flow (air), in particular for ventilation purposes. To prevent the oil from being carried out of the enclosures by the gas flow, the gases are evacuated into “oil separators” generally formed by radial chimneys arranged in the low-pressure shaft, the oil being captured on the walls thereof and returned to the corresponding enclosure by centrifugal force. The oil separators communicate with a degassing tube (also rotary) concentric to the low-pressure shaft and in the enclosure of which the gases are carried from the oil separators to the outlet of the degassing tube where they are ejected, generally around the turbojet nozzle.
The upstream and downstream enclosures are formed and delimited by the walls of the stationary structure of the turbojet, but also by the walls of the rotary elements. They must enable the passage of a gas flow, but retain as much oil as possible therein, and for this reason the seal between the stationary elements and the rotary elements of an oil enclosure is a particularly delicate problem.
Traditionally, the seal is effected using a labyrinth joint, i.e. formed by ribs rigidly connected to a rotary part and an abradable material rigidly connected to a stationary part against which the ribs rub. This rubbing occurs with a given clearance to enable the passage of a gas flow coming from the low-pressure or high-pressure compressors; these gases oppose the egress of oil through the labyrinth joint; the flow rate thereof is dimensioned to be sufficient at slow speeds and is therefore excessive in other flight phases (in which the flow rate of the air aspirated by the fan of the turbojet is greater). This excessive flow in the other flight phases has at least two detrimental effects: firstly, it proportionately reduces the efficiency of the engine and, secondly, it tends to draw a greater quantity of oil out of the enclosure, around the oil separators.
It has therefore been envisaged to replace the labyrinth joints with “brush” seals, i.e. having a plurality of juxtaposed, substantially radial fibers that are attached to a stationary part, the free extremities of which are in contact with a rotary part (or very close thereto), the fibers being preferably slightly inclined in the direction of rotation of the rotary part; the fibers of the brush seal may for example be made of carbon. Such a device is in particular described in patent application US 2004/0256807 filed by General Electric.
Such brush seals have the advantage of needing to be traversed by a gas flow having a flow rate that is not too high to guarantee the oil seal thereof On the other hand, they have the drawback of tending to cause coking of the oil they come into contact with. Coking is the transformation of oil into a solid deposit; it is caused by reheating oil stuck to the carbon fibers; it reduces the effectiveness of the brush seal. Furthermore, the rubbing of the bristles on the track of the rotary part designed to touch the extremities thereof causes them to wear and therefore also reduces the effectiveness thereof over time.